a. Abstract The transcriptional coactivator PGC1? was discovered by my group in 1998. It functions as a dominant regulator of mitochondrial biogenesis and oxidative metabolism by coactivating several nuclear transcription factors that control the broad program of mitochondrial gene expression. PGC1? also has important tissue specific functions, including control of adipose thermogenesis, the fasting response in liver, and mitochondrial biology and resistance to atrophy in skeletal muscle. Mechanisms that activate thermogenesis in fat and prevent atrophy in muscle are of enormous importance in human metabolic diseases such as diabetes and obesity. Preliminary data illustrates a very robust and novel translational control of PGC1? mRNA in cultured cells and in vivo; this mRNA translation is regulated by insulin and IGF1 signaling through AKT and mTORC signaling. Moreover, it is negatively regulated by the presence of a very small open-reading frame (uORF) just upstream of the codon that begins translation of the canonical PGC1?1 (the canonical PGC1? isoform; hereafter just called PGC1?) mRNA. Loss of this uORF by deletion or mutation increases the translation of PGC1? mRNA while ablating the insulin/IGF1 effect. This uORF encodes a predicted peptide of 15 amino acids that is strongly conserved in all mammalian species. We will begin these studies by using several mouse models using CRISPR technology (now created) which increase or decrease expression of this uORF by altering the start codon of this small encoded peptide (Aim 1). Mice will be analyzed for effects on key aspects of animal metabolism and physiology (Aim 2). These will include energy expenditure and resistance to obesity-linked glucose intolerance via thermogenic fat, gluconeogenesis in liver and exercise tolerance in muscle. Since skeletal muscle and its atrophy is a critical component of aging and an important target of insulin action, we will examine atrophy in the muscle-selective models. Mechanisms by which the 5' UTR and uORF control translation of PGC1? mRNA will be examined in cells by determining if the uORF functions in cis or trans via 2 plasmid experiments and through use of molecular ?toeprint? and ?footprint? assays (Aim 3). The presence of the uORF peptide in cell extracts will be determined by Mass Spectrometry with the use of synthetic ?heavy? peptides as key internal standards. Moreover, we will set up an in vitro translation system and determine if this regulation can be recapitulated in vitro. Key regulatory components of this system will be isolated by established affinity chromatography methods using oligonucleotides. Finally, Aim 4 will address the critical question of how insulin/IGF1 signaling impacts this translational control through quantitative phosphoprotein Mass Spectrometry in insulin treated cells. Phospho-proteomic analyses will also be applied to components isolated through the affinity methods described above. Together, these data will provide crucial perspectives and potential new therapeutic targets through which mitochondrial biology, physiology and disease processes might be manipulated in in vivo settings.